1. Field of the Invention
The present invention relates to a MOS transistor adapted for use particularly as a drive MOS transistor in a source follower circuit which constitutes a charge detector employed in an output of a charge transfer device.
2. Description of the Prior Art
In a CCD solid-state image sensing apparatus or the like using a charge transfer device, a floating diffusion type charge detector is employed in its output circuit. As shown in FIG. 5, such floating diffusion type charge detector has a source follower stage 51 for converting into a signal voltage the signal charge supplied from a horizontal shift register (charge transfer device) in the CCD solid-state image sensing apparatus to a floating diffusion region FD.
It is noted of late that the demand for a higher sensitivity of the CCD solid-state image sensing apparatus has been increasing year after year, and enhancement of the conversion efficiency in the charge detector is essential in realizing such higher sensitivity. For achieving a raised conversion efficiency of the charge detector, some requirements need to be satisfied including a reduced gate area in the drive MOS transistor Q1 of the source follower sage 51, i.e., a dimensional reduction of the drive MOS transistor Q1.
Generally a MOS transistor is produced, as shown in FIG. 6, by first growing a field oxide film (SiO.sub.2) 12 to a thickness of several 100 nm on a silicon substrate 11, then etching predetermined portions for source, gate and drain electrodes, and after forming a gate electrode 14 of polysilicon, implanting impurity ions of a high concentration therein to form desired source and drain electrodes.
In such MOS transistor, its channel length L is determined by the width of the gate electrode 14 of polysilicon, while the channel width W is determined by the bottom width of the field oxide film 12, so that in the final product, the dimensional variation is rendered greater in the channel width W. Accordingly, with reduction of the gate area in the drive MOS transistor Q1 of the source follower stage 51 for the purpose of enhancing the conversion efficiency of the charge detector, such variation range comes to affect the operation (I.sub.D -V.sub.D curve) of the MOS transistor to consequently vary the DC bias of the output voltage from the source follower stage 51, hence increasing the nonuniformity of the conversion efficiency.